The present invention relates to a brake system with electronic anti-lock control and traction slip control for use with vehicles, in particular for road vehicles. Such systems are equipped with a power-assisted dual-circuit braking pressure generator connected to hydraulic circuits which are connected to one front wheel and one rear wheel, in diagonal brake-circuit allotment and an auxiliary-pressure supply system which can be switched on during the control phases. On brake application, the auxiliary pressure supply system generates a controlled auxiliary pressure proportional to the pedal force and during traction slip control, generates an uncontrolled auxiliary pressure, limited by the pump output or by a pressure relief valve. Valves control the braking pressure in the wheel brakes and connect the braking pressure generator and/or the auxiliary-pressure supply system to the wheel brakes. Wheel sensors and electronic circuitries determine the wheel rotational behavior and generate electronic braking-pressure control signals.
One such proposed brake system publishes (patent application P No. 35 27 190) provides electronically controlled brake slip and traction slip in this manner. The braking pressure generator is a tandem master cylinder, connected upstream of which is a vacuum power brake booster and downstream of which are the two hydraulically isolated brake circuits with one front wheel and one rear wheel connected in diagonal allotment. Moreover, the system is provided with a hydraulic auxiliary-pressure supply system which generates a pedal-force-proportional auxiliary pressure for brake slip control and an uncontrolled auxiliary pressure for traction slip control. A relatively complicated valve assembly hydraulically isolates the braking pressure generator in the control phases and instead, connects the auxiliary-pressure supply system to the wheel brakes. The valve assembly includes an inlet valve/outlet valve pair which permits control of the braking pressure in one diagonal of each brake circuit. It further includes another inlet valve, i.e. a valve open in its inoperative position, in the connections leading to the non-driven wheels. Because of this additional valve, the coupling of the braking pressure variation in the two wheels connected to one hydraulic circuit is reduced or temporarily eliminated during the anti-lock control phase. Above all, this valve enables a traction slip control in such dual-circuit brake systems, because the pressure fluid conduit leading to the non-driven wheel can be interrupted and, thereby, only the driven wheel can be acted upon by braking pressure.
In other proposals concerning the design and the connection of such a valve assembly one started out from the consideration that the auxiliary pressure must be controlled by means of changeable multi-directional control valves in addition to the inlet valve/outlet valve pairs for the braking pressure control U.S. patent application Ser. No. 069,770.